Ensure order association is set correctly when adding admin adjustments

app/controllers/spree/admin/adjustments_controller.rb

@@ -5,6 +5,7 @@ module Spree
       destroy.after :reload_order
 
       prepend_before_action :set_included_tax, only: [:create, :update]
+      before_action :set_order_id, only: [:create, :update]
       before_action :set_default_tax_rate, only: :edit
       before_action :enable_updates, only: :update
 
@@ -22,6 +23,10 @@ module Spree
         parent.all_adjustments.eligible.find(params[:id])
       end
 
+      def set_order_id
+        @adjustment.order_id = parent.id
+      end
+
       # Choose a default tax rate to show on the edit form. The adjustment stores its included
       # tax in dollars, but doesn't store the source of the tax (ie. TaxRate that generated it).
       # We guess which tax rate here, choosing: